FILE_RESERVE_OPFILTER flag undocumented

Hi,

I am writing mini filter driver. I am creating hard link to file when file
delete request come in IRP_MJ_SET_INFORMATION as FileDisposiionInformation as
information class.

While creating hardlink using FltSetInformationFile(), this routine issues
create request with createoption as " FILE_RESERVE_OPFILTER and
FILE_WRITE_THROUGH " flag set.

What is meaning of FILE_RESERVE_OPFILTER flag ? It is not documented.

Why create request come to mini filter driver when we create hard link by
FltSetInformationFile() call ?

Thanks in Advance